I am quite surprised that no one has mentioned a baseball bat or a chain, as they are both very common items.
One would require very little imagination to use a bat as an "improvised" weapon. Any attacker (knife-wielding or otherwise) would undoubtedly think twice about assaulting someone who is in possession of a baseball bat (unless, of course, the attacker also has a bat). Since the sole purpose of a baseball bat is to strike things (such as baseballs), and since the human head is roughly globular in shape (like a rather large baseball), the baseball bat is the perfect "improvised" bludgeoning weapon. They (the bats) are quite readily available, relatively inexpensive, are specifically designed to comfortably fit in the human hand, and come in a variety of colors, styles, and materials (I suggest one made of wood, as it produces a satisfying crack upon impact with a hard surface; aluminum bats tend to make an awful "pinging" sound). And, best of all, the baseball bat is legal to carry almost everywhere in America.
As for chains, these require
slightly more imagination to be used as an improvised weapon, but are (depending on length, size, and weight) just about as effective as a bat. One would most likely swing a chain at an attacker, hoping to hit a vital area, and (depending on their skill with the chain) might possibly disarm the assailant in the process. A chain can be used to entangle an attacker if it can be wrapped around his arms or legs. A chain could also be used as a garrote. One particular advantage of using a chain is that it, as a whole, has no specific shape, and can therefore be stored in a variety of places (in a glove compartment, tool box, car trunk, coat pocket, or perhaps dangling from some idiot's baggy black pants). As with bats, chains are relatively inexpensive, widely available, come in a great variety of lengths, sizes, colors, materials, and designs. For the most part, it is legal to possess a chain almost everywhere.
In a fight, I would personally prefer a chain (or possibly a bat) as an "improvised" weapon, rather than a knife (although, if I possesed a horseshoe crab, I would use it simply for the novelty of doing so). The reach of a bat or chain is greater than that of a knife, and a strong person can do a great deal of damage with either one. The simple, brutal efficiency of a chain or bat should not be overlooked. Even so, one must never forget to carry a good knife.
(As a side note, I would like to pose a question about the legality of possessing a horseshoe crab for use as a defensive weapon. Is there any licensing or paperwork required? Must the crab be holstered and concealed, or carried in the open? Is there special training involved?

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TheSurvivalist